1. Field of the Invention
The present invention relates to antenna duplexers mounted on portable telephones, etc.
2. Description of Related Art
As shown in FIG. 3, an antenna duplexer 80 comprises an antenna terminal ANT to be connected to an antenna, a transmitting signal terminal Tx to be connected to a transmitting circuit, and a receiving signal terminal Rx to be connected to a receiving circuit. The antenna terminal ANT is connected to the transmitting signal terminal Tx and the receiving signal terminal Rx via a branchpoint 10. A transmitting filter 51 comprising a surface acoustic wave device is interposed on a signal line connected to the transmitting signal terminal Tx through the branchpoint 10 while a receiving filter 41 comprising a surface acoustic wave device is interposed on a signal line connected to the receiving signal terminal Rx through the branchpoint 10 (see JP-A-340781/1999).
Furthermore interposed between the branchpoint 10 and the receiving filter 41 is a phase-matching strip line 7 for phase rotation, to match the phases between the transmitting filter 51 and the receiving filter 41 (see JP-A-307383/2000). Incidentally the phase-matching strip line 7 is interposed between the branchpoint 10 and the filter having a higher pass band of the transmitting filter 51 and the receiving filter 41.
FIG. 4 shows the construction of the antenna duplexer 80 which is enclosed in a package. The transmitting filter 51 and the receiving filter 41 are mounted on a surface of a multilayer ceramic package 82. Arranged into two rows in the surrounding area of the filters 51, 41 are a transmitting signal input pad 9a and a transmitting signal output pad 9b to be connected to input-output terminals of the transmitting filter 51, a receiving signal input pad 9c and a receiving signal output pad 9d to be connected to input-output terminals of the receiving filter 41, and a plurality of grounding pads 91 (see JP-A-340781/1999).
With the antenna duplexer, when a signal is to be transmitted, the signal received by the transmitting signal terminal Tx passes through the transmitting filter 51 and thereafter is fed from the antenna terminal ANT to an antenna. In this case there occurs a phenomenon that the signal received by the transmitting signal terminal Tx sneaks around to the receiving signal terminal Rx, so that there is a need to take sufficient isolation measures against the phenomenon.
Accordingly with the conventional antenna duplexer, various isolation measures are heretofore taken to a signal path from the transmitting signal terminal Tx to the receiving signal terminal Rx via the transmitting filter 51 and the receiving filter 41. However, the conventional antenna duplexer has not yet achieved the sufficient isolation effect, and therefore there is still room for more improvement.